Many devices and systems include various numbers and types of sensors. The varied number and types of sensors are used to perform various monitoring and/or control functions. Advancements in micromachining and other microfabrication techniques and associated processes have enabled manufacture of a wide variety of microelectromechanical (MEM) devices, including various types of sensors. Thus, in recent years, many of the sensors that are used to perform monitoring and/or control functions are implemented using MEM sensors.
One current method that is used to form a MEM device makes use of a modified wafer known as a silicon-on-insulator (SOI) wafer. As is generally known, an SOI wafer is essentially a silicon wafer having a sacrificial layer disposed thereon, and an active, single-crystalline silicon, layer disposed on the sacrificial layer. The active layer is masked, patterned, and selectively etched to define the basic structural features of the MEM device. This structure may then be further processed to define various features, such as metal interconnects or isolation structures. After the all structural features are defined, portions of the structure may then be undercut by removing portions of the sacrificial layer below the structure, thereby releasing those structure portions.
The above-noted release process may be implemented as either a single-step etch process or a two-step etch process. With a single-step etch process, the device undergoes a single etch process until full structure release occurs. If a two-step process is used, during the first step, the device undergoes a first etch process that results in only a partial structure release. During the second step, the device undergoes a second etch process that results in a full structure release.
One particular type of MEM sensor that is used in various applications is an accelerometer. Typically, a MEM accelerometer includes, among other component parts, a seismic mass that is resiliently suspended by one or more suspension springs. The seismic mass and suspension spring may each include one or more etch openings that allow an etchant to reach, and thereby remove, the sacrificial layer during the release process. Preferably, the seismic mass includes a plurality of etch openings that are sized and spaced to facilitate the undercut of the seismic mass, whether a single-step or a two-step release process is used.
In high sensitivity (e.g., high aspect ratio) accelerometers, the suspension springs are relatively long and thin, and the seismic mass is relatively large. One method that may be used to increase the seismic mass is to reduce the number, and increase the spacing between, the etch openings that are formed therein. Thus, in many instances, whether a single-step or a two-step release process is used, the suspension spring may be released before the seismic mass is released. When this occurs, the suspension springs can be susceptible to stiction, which can render the MEM accelerometer inoperable.
Hence, there is a need for a MEM device, such as a high aspect ratio accelerometer, and a method of making the same, that reduces the susceptibility thereof to stiction. The present invention addresses at least this need. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.